JuliaDB flatten multiple columns

General Usage
3

OK, with foldl this actually works for any number of groups:

julia> t = table((group = [1,1,2,2,3,3], x = [1,2,3,4,5,6]))
Table with 6 rows, 2 columns:
group  x
────────
1      1
1      2
2      3
2      4
3      5
3      6

julia> t2 = groupby(t, :group, usekey = true) do k, r
           (x2 = r.x .+ k.group, y2 = rand(length(r)))
           end
Table with 3 rows, 3 columns:
group  x2      y2
────────────────────────────────────
1      [2, 3]  [0.0581229, 0.019784]
2      [5, 6]  [0.63852, 0.855233]
3      [8, 9]  [0.023636, 0.783005]

julia> foldl(merge, table.(rows(select(t2, Not(:group)))))
Table with 6 rows, 2 columns:
x2  y2
─────────────
2   0.0581229
3   0.019784
5   0.63852
6   0.855233
8   0.023636
9   0.783005